Electrical strain gages are frequently utilized for the measurement-technology evaluation of forces or for the monitoring of mechanically loaded structural components. The strain gages detect the strain of structural elements impinged or acted on by force. Such electrical strain gages usually consist of photolithographically produced meander-shaped measuring grids of an electrical resistance material that is applied on a support film of synthetic plastic and is usually covered with a further synthetic plastic protective film for mechanical protection. These electrical strain gages are applied on a deformation body for the detection of a load-dependent strain and convert the strain, through a resistance variation of the measuring grid, into an electrical signal that is proportional to the strain or the force influence.
Normally strain gages that are embodied as so-called rosettes are utilized for the biaxial measurement of strains. In that regard, usually two or three individual measuring grids are arranged on a common support film, which are usually offset from one another at 45°, 60° or 90° angles. Such strain-measuring rosettes are mostly used for measuring the magnitude of the strain or force along and perpendicular to a main axis or to determine the orientation of the main axis. In that regard, T-rosettes with two measuring grids are known, which are arranged offset 90° relative to one another. These are predominantly utilized for this if a biaxial stress condition exists, of which the main direction is known. Rosettes with three measuring grids are mostly used for determining a biaxial stress condition of which the main stress directions are unknown. In that regard, the measuring grids must be arranged as near as possible to one another in order to be able to detect the same strains at the same location, whereby only therewith an exact measurement can be ensured. Therefore, such known rectangular rosettes with three measuring grids are rarely larger than 10×20 mm. However, such electrical strain gages are very sensitive with respect to electromagnetic fields or high voltage related influences and also may not be utilized in areas subject to the danger of explosion.
Such a sensor for the high voltage and electromagnetically insensitive detection of biaxial mechanical stresses is known from the EP 1 129 327 B1, which optically determines the strains to be measured. For that purpose optical waveguides are provided, which consist of optical fibers. So-called Bragg gratings are written or introduced into these optical fibers, and these Bragg gratings produce a reflection wavelength that is proportional to the detected strain. These optical fibers with impressed Bragg gratings are embedded in a support layer of epoxide resin or adhesively bonded onto plates. This support layer can then be secured onto the surface of deformation bodies and thus transmits the strain acting on this support layer onto the strain-measuring Bragg gratings. Due to a strain, the reflected Bragg wavelength varies or changes corresponding to the strain and can be detected. This optical strain sensor is embodied as a rosette for the measurement of a biaxial stress and consists of a light waveguide with at least two or three Bragg gratings that are arranged one behind another, and that are oriented at angles of 45°, 60° or 90° relative to one another, and thereby can detect the strains of deformation bodies like electrical strain gage rosettes. In that regard, the connection sections of the waveguide between the Bragg gratings are guided in a curve or arc shape and may not fall below certain radii of curvature due to the reflection losses. For minimizing the radii of curvature it is additionally suggested to strongly taper the bent or curved connection sections, since thereby the reflection losses can be reduced. However, the connection sections of the waveguides between the Bragg gratings are apparently still considerably longer than the strain-measuring Bragg gratings themselves, which already require a length of approximately 10 mm. Therefore, such rosettes of optical fibers with two or three Bragg gratings on a common support layer, with comparable accuracy, still require a considerably larger application surface than is required with electrical strain gage rosettes.